Overview

Our project, funded as part of the NSF 2010 Project, will identify the genes required to construct exine, the outer wall of pollen grains. This wall contains sporopollenin, a complex biopolymer with an unknown chemical composition -- sporopollenin's unparalleled strength and chemical resistance is critical for pollen survival. Exine also contains molecules that play an important role in the earliest stages of pollen recognition, enabling the female stigma cells to capture the most appropriate pollen grains. Pollen-stigma binding occurs within seconds of contact and is mediated by exine components that are unusually stable and resistant to harsh chemical treatments. Exine-mediated adhesion either requires proteins with unusual properties or other non-proteinacous molecules sufficiently diverse to confer species-specificity.

Exine constituents are primarily produced by anther tapetal cells, transported to the anther locule, and assembled into highly patterned structures on the pollen surface. The inert and irregular nature of exine has confounded chemical analysis, but recent Arabidopsis genetic surveys are more promising, revealing genes and pathways required for exine structure and function. This work will extend these efforts, defining genetic networks required for exine biosynthesis. It will assess the roles of specific genes in exine assembly, patterning and adhesion, and will sort these genes into genetic and metabolic pathways.

(A) Pollen from different species has remarkably different exine morphology. Scanning electron micrographs of several pollen types, including Arabidopsis, Ambrosia, maize, Plumbago, and Artemisia douglasiana, which vary in size, exine ornamentation, and number and arrangement of pores.	(B) Structure of the complex pollen wall of Arabidopsis pollen. Transmission electron micrograph of cross-section of Arabidopsis pollen. P- pollen grain cytoplasm, i- intine, e- bacula of exine, pc- pollen coat.